Controller modules and systems for converter units in wind turbines require accurate operation while keeping a good time response.
For measuring the rate of change of frequency prior art documents disclose analog and digital measurement alike. US003032715 describes an analog frequency measurement by CCU with structural code. GB2159963 describe a measurement system based on counting the period of the voltage waveform as used in state of the art relays for measuring the rate of change of frequency.
The use of relays for measuring parameters relating to frequencies are well known. In electrical systems, a rate of change relay detects the zero crossings of a generator voltage. The relay then measures the time between the zero crossing and calculates a new frequency after each zero crossing. In case that the frequency changes too much the relay will trip. This method is, however, very sensitive to noise in the voltage waveform and typically requires several voltage waveforms before a measured can be determined. Moreover, relays are conceptually designed to detect vector shift rather than the rate of change of waveforms.
Other known methods are based on frequency measurements. For instance, US2007136013 describes several frequency measurements that allow the calculation of the finite derivative of said frequencies. This document describes a finite method with measuring points spaced in half cycle periods. With said measurements as inputs, the method subsequently determines the time derivative by subtracting two consecutive measurements and dividing the result by the half period, or eventually the inverse of the frequency value as measured.
Another method known to the art is to employ the frequency which is measured by the converter unit using state of the art phase-locked loops (PLLs). Those phase-locked loops calculate angle and frequency of the voltage waveform with a good accuracy. Subsequently, the method for calculation of the rate of change of frequency (ROCOF) will be based on the derivative of the frequency measurement from the control converter unit PLL. Mathematical considerations for extracting said outputs are mainly based on two methods, either calculating the derivative of the grid voltage frequency values using finite differences, or by calculating the second derivate of the grid voltage angle values using derivate functions.
When measuring the rate of change (ROCOF) noise a critical issue. This noise comes from the grid voltage and also from harmonics of the grid voltage. Furthermore, the derivative nature of ROCOF causes noise to be amplified and even worsens said critical issue.
Hence, an object of the present invention is to provide a measuring method and system that is robust and not affected by noise. Encompassed within the framework of noise elimination strategies, several other problems may arise, for example the discontinuity of measured output waveforms.
Controller units of wind turbines need to measure frequency and rate of frequency of variables, preferably voltage waveforms in converter units of wind turbines. The operation of said controllers requires a quick time response, typically a range between 400 to 200 ms or even less. This is difficult to attain while keeping a good accuracy at the same time.
In wind turbine systems, there is a trade-off between the accuracy and time response of ROCOF. Those two requirements are generally contradictory because increasing accuracy requires filtering which also increases time response.
Hence, it is yet another object of the present invention to provide a measuring method and module that can be accurate and yet have a quick time response for controlling the converter unit of a wind turbine.